1. Field of the Invention
The present invention relates to a zoom lens device and, in particular, to a zoom lens device which is ideal for use in a video camera or the like.
2. Description of the Related Art
Conventionally, there has been known a zoom lens device using a zoom lens barrel having a zooming mechanism of a taking lens which moves back and forth first and second zoom lens groups to thereby perform a zooming operation. In such zoom lens barrel, a connecting pin is planted on a cam barrel which is rotatably supported inside of a fixed barrel, the connecting pin is fitted into a zoom ring, and the zoom ring is then rotated to thereby achieve the zooming operation. Gears, which are used to transmit a force from a motor, are formed integrally with the zoom ring. In this type of zoom lens barrel, it is necessary to form a clearance groove in the fixed barrel supporting the cam barrel in such a manner that the groove corresponds to an angle of rotation of the connecting pin.
On the other hand, in the case when a zoom mechanism of a high magnification is used, the quantity of lens groups to be moved by a cam barrel is increased as the magnification becomes higher. In general, the relative positions of first and second zoom lens groups are determined in an optical design manner. Then, in order to increase the quantity of movement of the lens groups with the same cam barrel diameter, an angle of the rising of a cam groove must be great. However, if this angle is great, then a force for driving the lenses is increased and also a quantity of shifting of the zoom lens group in the optical axis direction due to play between a groove and a pin fittable into the groove is increased.
Therefore, conventionally, the angle of the groove in the cam barrel must be as small as possible. In this situation, in order to make the angle small to thereby secure a great quantity of movement, the angle of zoom rotation must be great or the diameter of the cam barrel must be increased to thereby increase the length of the cam groove, which gives rise to a problem that the zoom lens barrel itself is large in size.
Also, in the above-mentioned conventional zoom lens barrel, a guide hole and a recessed portion for prevention of swinging are formed in the first moving lens frame, whereby a first guide bar is fitted into the guide hole to guide the first moving lens frame and a commonly used swing-preventive bar is fitted into the swing-preventive recessed portion to prevent the swinging of the first moving lens frame. Further, a guide hole and a recessed portion for prevention of swinging are formed in the second moving lens frame, whereby a second guide bar is fitted into the guide hole to guide the second moving lens frame and the above-mentioned common swing-preventive bar is fitted into the swing-preventive recessed portion to prevent the swinging of the second moving lens frame. In addition, in the second moving lens frame there is provided a recess for prevention of interference with the first guide bar, and in the first lens frame there is provided a recess for prevention of interference with the second guide bar.
However, in the above-mentioned conventional zoom lens barrel, the swing-preventive bar is required besides the first and second guide bars and the recessed portions for prevention of interference must be worked in the first and second lens frames, respectively. Due to this, the conventional zoom lens barrel is disadvantageous in that it requires a large number of parts and a great number of man-hour for working.
In a zoom lens barrel used in a conventional video camera, in general, a front-lens focusing system is employed and an iris diaphragm is fixed between a zoom lens group and a master lens group.
However, when the front-lens focusing system is employed, a front lens is drawn out when focusing is executed and, therefore, in order to obtain a great angle of view on the side of a wide angle lens, the diameter of the front lens must be greater and thus the weight of the whole video camera becomes heavier. Also, when the iris diaphragm is interposed and fixed between the zoom lens and master lens, the zoom lens barrel must be larger in size in order to obtain a high magnification.
A focus lens of an automatic focusing system is driven and brought into focus by a motor in accordance with an AF signal. In such drive mechanism for driving the focus lens of an automatic focusing system, for example, the output shaft of the motor is connected to a threaded shaft with which a nut member is threadedly engaged. In such structure, if the threaded shaft is rotated, then the nut member is also rotated together with the threaded shaft and, for this reason, restriction means must be provided in order to prevent the rotation of the nut member. For example, there is known rotation preventive means in which a pin is planted on and projected from the nut member and the pin is fitted into an elongated groove formed in a main body of a video camera or the like to thereby prevent the rotation of the nut member.
On the other hand, if there exists any play between the nut member and the lens frame, then the back-and-forth movement of the lens frame cannot be achieved with accuracy. However, if no play is present therebetween, then there arises another problem; that is, there are required position precision conditions such as parallelism between the lens frame and the nut member and the like. Such precision becomes severer as the quantity of the movement of the lens frame is increased. As an example of the solutions to such problem, there is known a method in which such play is biased by a torsion spring to thereby absorb the positional shifting between the lens frame and the nut member while the lens frame can be moved back and forth with accuracy.
In the above-mentioned structure, if the threaded shaft is rotated, then the nut member is moved back and forth and, with the back-and-forth movement of the nut member, the moving lens frame is moved back and forth along the guide bar. Since the nut member and moving lens frame are in engagement with each other with the pin and projection piece being in contact with each other, the shifting occurring between the threaded shaft and the guide bar of the moving lens frame can be absorbed.
However, due to the need of the torsion spring, the above-mentioned conventional lens drive device has a problem that it is not easy to assemble.
Conventionally, there is also known a zoom lens barrel which includes a zoom lens drive mechanism and a focus lens drive mechanism. The zoom lens drive mechanism includes a moving lens frame which can be guided and thus can be moved back and forth by a guide bar to perform a zooming operation. On the other hand, in the focus lens drive mechanism there is provided a moving lens frame which can be guided and moved back and forth by a guide bar to perform a focus adjustment.
However, in the conventional zoom lens barrel, the moving lens frame of the zoom lens system and the moving lens frame of the focus lens system are guided by separate guide bars, respectively. Due to this, the conventional zoom lens barrel is disadvantageous in that it has some trouble in adjusting the optical axes of the zoom and focus lens systems. Also, the separate provision of the guide bars in the focus and zoom lens systems results in the increased number of parts.
In an ordinary taking lens barrel, there are used parts which are referred to as glittering members. For example, in a taking lens barrel of a type that rotates a cam barrel to thereby move a zoom lens back and forth, there are employed a follower pin, which is planted on and projected from a lens frame and can be fitted into a cam groove in the cam barrel, a guide bar provided in the lens frame and the like, all of which are plate treated and thus are easy to glitter.
In view of this, in the above conventional taking lens barrel, the inner surface of the barrel is coated with paint for flattening and also a light shield surface is formed in the inner surface of the barrel to prevent the reflection of the barrel inner surface. However, this results in the complicated working of the inner surface of the taking lens barrel.